This invention relates to electronic article surveillance systems and, in particular, to EAS systems using radio frequency (RF) signals.
U.S. Pat. No. 4,063,229 discloses an EAS system in which RF signals are used to detect the presence of tags in an interrogation zone. In the system of the '229 patent, an RF signal at a predetermined RF carrier frequency is transmitted into the interrogation zone. Each tag in the zone which receives the transmitted RF signal develops and transmits an RF tag signal based thereon. A receiver in the system is responsive to RF signals and processes the RF signals in an attempt to evaluate whether the signals contain an RF tag signal. If the receiver evaluation is that a tag signal is present, an alarm signal is produced indicating the presence of a tag in the zone.
In the '229 patent, one form of the system utilizes RF signals in the microwave frequency range and, in particular, utilizes a microwave carrier frequency at 915 MHz. Each tag in the system, in turn, includes a nonlinear or mixing element which produces a RF tag signal at twice the carrier frequency, i.e. at 1830 MHz.
The RF signals received at the receiver are mixed or compared with a reference signal, i.e., an 1830 MHz signal. If a tag signal is present, a further lower frequency RF signal, i.e., a 30 MHz signal, indicating the presence of the tag signal is produced. This lower frequency signal can then be detected and an alarm signal generated.
Other EAS systems of the RF type utilize two transmitted signals, one an RF signal at a predetermined microwave frequency and a second a modulated signal at a predetermined intermediate frequency (IF). In this type of system, a tag in the interrogation zone receives both the RF signal and the modulated IF signal and mixes the signals. The mixed signals then form an RF tag signal which is transmitted or reradiated by the tag. At the receiver, the received RF signals are also mixed this time with a signal at the RF carrier frequency of the transmitted RF signal.
This mixing produces a mixed signal which contains frequencies indicative of the modulated IF signal content of any RF tag signal which might be present in the received RF signals. The mixed signal is then demodulated to extract any signal content in a frequency band which includes the modulation frequency of the transmitted IF signal. The latter signal content is compared with a signal at the modulation frequency and depending upon the result of the comparison an alarm signal is generated. Systems of this type using RF and IF signals and tags for these systems are disclosed, for example, in U.S. Pat. Nos. 4,139,844, 4,642,640, 4,736,207 and 5,109,217.
All the above EAS systems are subject to interference from sources which transmit signals at or close to the RF frequencies being used in the systems. This interference can mask the RF signals being transmitted by the system transmitter as well as the RF tag signals being received at the system receiver. As a result, the sensitivity of the system is reduced.
Various techniques have been used to compensate for this interference. One technique involves increasing the power of the transmitted RF signal and another technique involves changing the carrier frequency of the transmitted signal. Both techniques, however, have their own disadvantages.
Increasing the power of the RF signal affords only a limited degree of compensation, since the power cannot be increased beyond that allowed by governmental regulations. Also, in order to provide increased power, the components of the system must be enlarged with an accompanying increase in cost. An increase in signal power may also result in signal transmission outside the desired interrogation zone, if the interference source is removed. Finally, increasing the power promotes an escalation of frequency band rivalry.
On the other hand, changing the RF carrier frequency of the transmitted RF signal usually requires that the crystal oscillator employed to generate the carrier be replaced with another oscillator operating at the new carrier frequency. This requires a service person to visit the site where the EAS system is located which is a costly procedure. Also, changing the crystal oscillator does not protect against a new noise source at the new frequency being encountered after the change is made.
It is therefore an object of the present invention to provide an EAS system and method which tend to avoid the above disadvantages.
It is a further object of the present invention to provide an EAS system and method in which interference is more readily avoided.
It is yet a further object of the present invention to provide an EAS system and method in which interference is avoided in a way which helps detect interference frequencies and/or allows operation near the edge of a permissible frequency band.
It is a further object of the present invention to provide an EAS system and method which result in less interference with other systems.